In recent years, the demand for compact optical systems increases with the popularity of portable electronic devices with photographing capability. Charge coupled devices (CCD) or complementary metal-oxide semiconductor (CMOS) sensors are two ordinary types for the photo-sensing device in an ordinary optical system. Furthermore, since the pixel size of sensors are reduced due to advances on the semiconductor manufacturing technology, and the present electronic device requires high performance, small size and lightness, Thus, a compact optical system with high image quality currently prevails over the market. For example, the conventional optical system, a three-lens assembly, implemented in the electronic device is disclosed in the specification of U.S. Pat. No. 7,145,736. With the advances of semiconductor manufacturing technology and the development of compact electronic devices, the image quality of compact optical system is highly required, but the conventional three-lens optical system may not satisfy the demand for better image quality. Accordingly, U.S. Pat. No. 8,089,704 discloses a four-lens assembly in the optical system. However, the distance differences among the lenses of the four-lens assembly results in the difficulty to minimize the size of the four-lens assembly. The refractive power of the first lens element affects the field angle of the optical system, so the region of image capturing is limited.
Therefore, it is necessary to develop an optical system with wide angle, high image quality and short total length.